This invention relates to the inhibition of formation of explosive compounds and conditions which can result from contacting concentrates of ammoniacal silver salts and strong alkali, which are normally utilized in dilute form, in simultaneous admixture with a reducer to effect electroless deposition of silver. A serious and inherent hazard encountered in shipment and use of ammoniacal silver salts, particularly in concentrated forms, is the danger of accidental or negligent admixture or contacting of the ammoniacal silver composition with a strongly alkaline material or a concentrated alkali solution, for example sodium hydroxide, potassium hydroxide or the like.
It is well known that upon such contact, without suitable safeguards or inhibitors, explosive conditions and compounds, often referred to as fulminating silver, can form and result in detonation, which can be quite violent and dangerous. This hazard is particularly prevalent in situations where for reasons of economy and commericial preferences many industrial firms utilizing these materials utilize concentrated solutions involving separate shipment and handling of the materials. Although the risk of formation of explosive compounds or conditions upon inadvertent or accidental admixture could be minimized by use and shipping of dilute solutions of ammoniacal silver salts and strong alkali, or by carefully controlling the admixture of the same together with a reducer to effect the desired electroless deposition, the safety hazard still remains, since shipping and plant personnel may not be fully informed regarding the dangers and human error is an ever present factor in any industrial endeavor.
It has long been known in the art that metallic silver can be electrolessly deposited by appropriate use of combinations of (1) ammoniated silver salts, hereinafter also referred to as "ammoniacal silver salts", (2) strong alkali, (i.e. metal hydroxides of Group IA which are strong electrolytes) such as sodium hydroxide or the like and (3) a reducer for the ammoniacal silver salts, such as carbohydrates, e.g. invert sugar, formaldehyde or the like, as well as more recently developed reducers such as aldonic acids and polyhydric alcohols, which will be discussed hereinafter in greater detail. It was soon discovered that explosions could occur with detonation resulting from even very slight mechanical or thermal stimulus in a mixture which could result from contacting concentrates of ammoniacal silver salts and strong alkali.
In the prior art it was recognized that such explosions were produced from reactions between silver diammine and hydroxyl ions, when brought into contact in aqueous solution. These ions are believed to react to first form silver amide, then silver imide and finally silver nitride. These substances are all black and rather insoluble in aqueous solution.
Of the foregoing compounds, the nitride is the most unstable and is capable of spontaneous explosion, as well as detonation upon the slightest mechanical or thermal stimulation, with the silver imide readily detonated by the explosion of the silver nitride. It is also known that formation of these explosive compounds is promoted by high alkalinity in the reaction medium and by increases in temperature. While it is possible to minimize the formation of such explosive compositions and conditions by maintaining either or both of the ammoniacal silver salt and a strong base in a highly diluted state, such dilute concentrations are not commercially economical, particularly with regard to the shipment of the materials from the supplier to the end user who utilizes the same to deposit metallic silver.
The sensitivity to explosion of the mixture formed by contacting an ammoniacal silver salt and strong caustic varies with time and temperature. However, it has been found that at normal room temperature, the critical concentration below which explosion is generally not likely to occur is about 45 g/l each of ammoniacal silver salt and strong alkali. Typical concentrates supplied for commercial use contain 250 g/l of ammoniacal silver and 200 g/l of sodium hydroxide, so that these materials must be diluted 5.5 and 4.4 times, respectively, in order to reach the 45 g/l critical level. Even at this critical level there would be no guarantee that explosion will not occur if the materials were admixed. Thus, further dilution would be recommended to provide a margin of safety. Such further dilution would be utterly impractical, however, because of high cost of transportation of the same weight of chemicals in up to ten times the amount of water now generally used.
Commercially, industrial use of ammoniacal silver salts and strong alkali is directed to concentrates of such solutions, which, if contacted, would result in a mixture having a concentration of perhaps 5 to 10 times the aforementioned critical concentration of avoidance of explosion simply on the basis of dilution. Accordingly, these solutions are manufactured, shipped, stored and handled in concentrated form until such time as they are prepared for appropriate reaction by simultaneous admixture to effect electroless deposition of silver on an appropriate work-piece surface.
In the event that ammoniacal silver salt and strong alkali are brought into contact in a mixture in which they are present above the aforementioned critical concentration, formation of explosive compounds and conditions can occur spontaneously, or over varying degrees of time, depending upon the resulting hydroxyl ion concentration (pH), the concentration of silver diammino ion and the temperature. For example, if one cc each of a 250 g/l solution of ammoniacal silver nitrate and a 200 g/l solution of sodium hydroxide are admixed, a black precipitate forms rapidly and a sharp explosion sufficient to disintegrate a test tube containing the mixture will occur rapidly, depending upon temperature.
Accordingly, there is substantial need for improved methods and compositions which will inhibit formation of such explosive compounds and conditions and allow concentrates of such solutions to be safely shipped, stored and utilized without risk of explosion.
The earliest known reducers for the electroless deposition of silver, e.g. carbohydrates, such as invert sugar, dextrose, fructose or arabinose will prevent formation of explosive compounds and conditions when admixed with an ammoniacal silver salt and strong alkali, because the silver salt is rapidly reduced to elemental silver and other non-explosive compounds. However, such reducers cannot be made available in the reaction medium by first being admixed with either the concentrated ammoniacal silver salts or the strong alkali, because such reducers in the presence of hydroxyl ions are catalytically converted to non-reducers and/or, in the presence of ammoniated silver, produce amounts of reduced silver sludge, only minute quantities of which would render the solution commercially unfit.
However, in U.S. Pat. No. 3,776,740 of Sivertz et al and Canadian Pat. No. 945,307 a method for electroless deposition of silver and method for inhibition of the formation of fulminating silver are disclosed, utilizing a class of reducing agents comprising aldonic acids having 4 to 7 carbon atoms. The preferred reducers disclosed by Sivertz et al are gluconic acid, sodium gluconate, sodium glucoheptonate or glucono-delta-lactone. This class of reducers, it was disclosed, could be included either with the ammoniacal silver salt, with the strong alkali, or a portion of the reducer with each of the concentrated solutions of ammoniacal silver salt and strong alkali. These reducers were found to be not only useful for deposition of metallic silver upon simultaneous combination of the solutions containing ammoniacal silver salts and strong alkali, respectively, but they were also found to have other properties, not in any known way related to their reducing capabilities, namely to resist the action of hydroxyl ions to destroy their effectiveness as reducers of silver ions. These reducers are also stable in combination with each of the various components.
As further disclosed in Canadian Pat. No. 945,307, this class of reducers, as a result of being stable in both concentrated solutions of ammoniacal silver salts and strong alkali could be added to the same to inhibit the formation of explosive fulminating silver upon accidental or inadvertent contacting, because the presence of the reducer would cause the silver salts to be rapidly reduced, thereby avoiding formation of explosive compounds or conditions in the mixture.
U.S. Pat. No. 3,983,266 discloses the utility of a broad class of polyhydric alcohols having from 3 to 8 carbon atoms as reducers of ammoniacal silver salts. The preferred polyhydric alcohol is sorbitol. However, while U.S. Pat. No. 3,983,266 recognized that such polyhydric alcohols could be used as reducers for ammoniated silver, its use as disclosed in that patent was specifically restricted to the long prior-established techniques for old conventional reducers, such as glucose, which required that such reducers must be applied from separate containers and in streams of spray entirely separate from the alkali and ammoniated silver. Accordingly, one skill in the art was taught by this patent that three concentrates were to be used, one of which was ammoniated silver, one sodium hydroxide and one reducer. Consequently, they would know that it was still necessary to deal most cautiously with the first two members of the three-part system, which are well known to be capable of producing explosives. U.S. Pat. No. 3,983,266 refers to Sivertz U.S. Pat. No. 3,776,740 and discusses the same as prior art, yet does not disclose or suggest that its own novel polyhydric alcohol reducers could be utilized in a two-part silvering system, such as that disclosed in the Sivertz patent for use with its aldonic acid reducers. In fact, U.S. Pat. No. 3,983,266 teaches against incorporation of sorbitol in strong alkali, thus precluding use of the reducer in the manner disclosed by Sivertz et al.
The present invention is particularly directed to the situation in which, though generally accidental or inadvertent, concentrated, solutions of ammoniacal silver salt and a strong alkali come into contact, resulting in the formation of a mixture of potentially explosive compounds, such as fulminating silver, or explosive conditions. In accordance with the present invention, not only is a method for inhibiting formation of such explosive compounds and conditions provided, but also an improved multi-part silvering system utilizing ammoniacal silver and strong alkali in conjunction with a suitable conventional reducer, is provided. This multi-part silvering system is protected against the formation of such explosive compounds and conditions as a result of inadvertent or accidental contacting of the materials, no matter in what proportion the concentrated solution of strong alkali and ammoniated silver happen to be mixed.